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Effect of Sand Production on Casing Integrity

This paper documents a case study about the effect of sand production on casing damage. Many wells were converted from production to water injection. Formation of precipitates across the injection interval required frequent well washing which in- advertently involved reducing the well pressure rapidly. Conse- quently, solids were released from the formation into the wells. The well washing and associated solids production went un- noticed for an unknown length of time until significant casing deformation was encountered during a recent workover. The sig- nificant solids production caused casing buckling near the perfo- ration interval. It also activated a weak plane in the overburden, causing further casing damage. This paper will present relevant field data and engineering analyses to support the above conclu- sions. Field measures to improve the casing’s resistance against the buckling are also described.

Introduction

This paper is concerned with casing integrity in conventional onshore reservoir production in the Niuzhuang area of the Shengli Oilfield in China. Worldwide experience shows that origins of casing failure are complex. Reservoir geology, drilling and cement practices, production drawdown schemes and casing hardware can all influence casing integrity. Nevertheless, major casing damage mechanisms can be summarized as follows: 1) chemical corro- sion(1, 2); 2) shear deformation along re-activated weak planes(3-8); 3) significant reservoir compaction(9-11); and, 4) excessive sand production(12-16).

Chemical corrosion is a major casing damage mechanism in the NiuZhuang field. In a total of six wells being investigated, damage to five of those wells was caused by corrosion(17). However, this paper describes another casing failure mechanism; unintended excessive sand production. The following description will first present relevant field data. Mathematical analyses are then carried out to quantify the damage mechanisms. Finally, field measures are proposed to correct the problem.

Reservoir Geology and Rock Mechanical Properties

Production in the NiuZhuang field mainly comes from two sand bodies at a depth of approximately 3,200 m. The reservoir is struc- turally simple with only a small normal fault at an offset of 10 m. The original reservoir pressure is abnormally high at 1.68 SG. Av- erage reservoir permeability based on core analysis is 24.5 mD and porosity is 18.2%. Porosity is the major fluid conduit and storage.

Natural fractures are not developed. Initial well production rate is very high, but the productivity decreases rapidly. Full production started in 1993. Water injection was needed in late 1994. The effect was seen at the production wells usually four to six months after water injection started at the surrounding wells.

No rock mechanical tests were done on our target reservoir rocks. Instead, rock mechanical properties were calculated from well logs and petrophysical data using Baker Hughes’ LMP (Log of Mechanical Properties)(11). Table 1 lists the average properties cal- culated at reservoir pressures equal to the peak injection pressure (70 MPa) or the minimum pressure reached during well washing (35 MPa). Table 1 also includes a set of lab-measured mechanical properties on a similar stratum in an adjacent oil field.

Well History and Casing Damage

Well N42 experienced significant casing deformation. It was completed in November 1990 and converted from production to injection four years later. Casing failure was encountered in early 2002; 11 years after its completion. Workover tools, and then pro- duction tubing, could not pass through the damaged interval due to dramatic multiple doglegs formed therein. Repeated efforts over 75 days could not free the tubing out of the casing. Locations of significant casing deformations are shown in Figure 1a. The failure occurred in two zones, all of which were below the cement return height. One interval is approximately 6 m long, shortly (about 5 m) above the perforation interval. As shown in Figure 1b, six dog- legs over this 6 m interval were felt during the workover. The other zone of significant casing deformation is located over a 9 m in- terval at approximately 300 m above the perforation interval.

Injection pressure at N42 was mostly at 70 MPa. However, to remove precipitates formed across the injection interval, the well was frequently washed via rapidly bleeding off the injection pres- sure by opening the wellhead to the atmosphere. This meant a rapid pressure drop of 35 to 40 MPa across the perforations during the well washing process. Note that the injection pressure was 70 MPa and the hydrostatic pressure at the perforation depth was 30 to 35 MPa. Rock fragments, including cement, were seen at the well- head. Four production tubing strings were buried in rock solids downhole when the casing failure was noticed. Therefore, it is reasonable to hypothesize that the near-wellbore rock around the perforation interval was aggressively disturbed. The rock is me- chanically weakened and then structurally loosened. The injected waste water likely reduced the rock strength due to swelling of the clay fines upon exposure to the freshwater. As a result, significant amounts of rock solids were produced over time. Open cavities were likely formed behind the casing, weakening or eliminating its lateral support. The casing is prone to buckle when the reservoir subsides in response to the pressure drops during the well washing.